The steel sheet to be used as a raw material for structural members in transport machines, typically automobiles, and various industrial machines is sometimes required not only to be excellent in such mechanical characteristics as strength, workability and toughness but also to have weldability in parts assembly and/or corrosion resistance during use. In order to generally improve the mechanical characteristics of a steel sheet, it is effective to render a steel sheet with fine microstructure. Therefore, a number of methods in order to render a steel sheet with fine microstructure have been proposed.
To sum up, the means for rendering a steel sheet with fine microstructure as known in the prior art include (i) the high reduction rolling method, (ii) the controlled rolling method, (iii) the alloying element addition method, and combinations of these.
The high reduction rolling method (i) is a technique which comprises employing a rolling reduction of about 50% or higher to cause accumulation of great strains in one rolling pass and then transforming austenite grains to fine ferrite grains or recrystallizing relatively large ferrite grains into fine ferrite grains caused by the great strains. Such technique makes it possible to obtain an ultra fine ferritic microstructure with a grain size of 1 to 3 μm by heating to a temperature not higher than about 1000° C. and then carrying out high-reduction rolling within a low temperature zone around 700° C. However, this method is difficult to realize on an industrial scale and, in addition, has a problem that since the fine ferritic microstructure readily allows grain growth during heat treatment, and the welded portion, upon welding, becomes softened or, upon hot-dip Zn plating, the mechanical characteristics expected are lost.
The controlled rolling method (ii) is a technique comprising carrying out multi-pass rolling generally at a temperature not lower than about 800° C. employing a reduction, per rolling pass, of not higher than 20 to 40%, followed by cooling. Various modifications have been disclosed, for example such as the method employing a rolling temperature within a narrow range around the Ar3 point, the method employing a shortened pass-to-pass time in rolling, and the method causing dynamic recrystallization of austenite while controlling the strain rate and temperature. However, no full investigations have been made concerning the cooling after rolling. While it is said that water-cooling immediately after rolling is preferred, the cooling, though said to be made immediately, is actually started after the lapse of 0.2 second or a longer period after rolling, and the cooling rate is at most about 250° C./second. Such method can reduce the ferrite crystal grain size of low-carbon steel simple in composition only to about 5 μm. Therefore, it is impossible to improve the mechanical characteristics to satisfactory levels.
The alloying element addition method (iii) is to render ferrite crystal grains fine by addition of a minute amount of at least one alloying element capable of suppressing recrystallization or recovery of austenite. Such alloying elements as Nb and Ti form carbides and/or segregate at grain boundaries to thereby prevent recovery and recrystallization of austenite, so that the austenite grains after hot rolling are fine and the ferrite crystal grains obtained by transformation of austenite are also fine. This alloying element addition method (iii) is often used in combination with the high reduction rolling method (i) and/or controlled rolling method (ii) mentioned above. This alloying element addition method (iii) is also effective in suppressing ferrite grain growth during heat treatment as well. However, this method has a problem in that it causes reductions in austenite volume fraction although it reduces the ferrite crystal grain size; further, the method is yet unsatisfactory in suppressing ultra fine ferrite crystal grains from growing during welding or hot-dip Zn plating. Therefore, the method is applicable only to limited steel species. In addition, the material cost goes up due to the alloying element(s) to be added.
A prior art referring to these high reduction rolling method (i), controlled rolling method (ii) and alloying element addition method is disclosed in Patent Document 1. In that document, a method is disclosed which comprises finishing working in one or more passes at a total reduction of 50% or higher within 1 second in a temperature range of Ar1+50° C. to Ar3+100° C. and carrying out forced cooling, after finishing of the working, within a temperature range of not lower than 600° C. at a cooling rate of not lower than 20° C./second.
Further, Patent Document 2 discloses a method which comprises carrying out rolling passes in at least 5 stands within the dynamic recrystallization temperature range, with the temperature difference between the first stand entry side and the last stand outlet side being not greater than 60° C.    [Patent Document 1] JP-S59-205447-A    [Patent Document 2] JP-H11-152544-A